When it comes to typical online forums signatures are a good way to promote things. Take a quote of HPMOR and attach a link to it.
Of course, you should only do this where the forum has made the foolish choice to allow signatures. (One of the things I appreciate about Reddit/LW compared to forums is how they strongly discourage signatures.)
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.